The present invention relates to a signal generating apparatus, and more particularly to a phase-locked loop based transmitter with a closed loop modulation compensation scheme, wherein the modulation compensation scheme calibrates a compensation filter according to an input voltage of a voltage controlled oscillator of the signal generating apparatus, and a method thereof.
Please refer to FIG. 1. FIG. 1 is a related art Sigma-delta fractional N phase locked loop (PLL) transmitter 10. The Sigma-delta fractional N PLL transmitter 10 comprises a phase locked loop circuit 11, a Sigma-delta modulator 12, a channel selector 13, a Transmit filter 14, and a compensation filter 15. The phase locked loop circuit 11 comprises a phase/frequency detector 11a, a charge pump circuit 11b, a loop filter 11c, a voltage-controlled oscillator 11d, and a divider 11e. If the transmitted signal of the Sigma-delta fractional N PLL transmitter 10 is the GMSK signal SGMSK, then the Transmit filter 14 is the GMSK filter. Furthermore, as the frequency response of the phase locked loop circuit 11 acts like a low pass filter, the compensation filter 15 is utilized to compensate the baseband data Sb before being modulated by the Sigma-delta modulator 12. The phase locked loop circuit 11 utilizes a reference frequency Sr to synthesize the required frequency to transmit the baseband data Sb. Furthermore, in a multi-band system, the phase locked loop circuit 11 needs to generate different frequency bands, and the channel selector 13 is utilized to select the required frequency band. Accordingly, through the variation of the dividing number of the divider 11e, the phase locked loop circuit 11 can generate different frequency bands accordingly. However, the loop bandwidth of the phase locked loop circuit 11 is sensitive to the variations of parameters in the transfer function of the phase locked loop circuit 11. Therefore, an effective scheme to calibrate the loop bandwidth of the Sigma-delta fractional N phase locked loop (PLL) transmitter 10 is necessary. For the full description of the related techniques, U.S. Pat. Nos. 7,103,337, 7,068,112, 6,724,265, and No. 6,806,780 can be referred to.